1. Field of the Invention
The present invention relates to a robotic arm end effector having an eddy current detector and, more specifically, to a mode switching circuit that allows a robotic arm end effector having an eddy current detector to operate in two modes concurrently.
2. Related Art
Because of the radiation hazard present within the pressurized water vessel of a nuclear reactor, maintenance and testing of components within the pressurized water vessel are typically performed by remote service devices, such as robotic arms. Such a service device typically includes a robotic arm which can generally access any point within the pressure vessel. The robotic arm will be fitted with an end effector capable of performing specific maintenance or testing tasks. For example, the water inlet and outlet of the nuclear reactor pressure vessel must be inspected, inter alia, for surface and near surface defects. Such inspections are performed utilizing a “sled” coupled to the robotic arm.
The sled has a frame to which inspection devices may be coupled. Inspection devices typically include ultrasonic probes and eddy current probes. Ultrasonic probes emit and/or receive ultrasonic frequencies. Thus, the ultrasonic probes send an ultrasonic pulse and are structured to detect the reflection thereof. That is, the ultrasonic pulse will reflect differently at a defect than at a generally smooth surface. Eddy current probes operate by detecting changes in a magnetic field. That is, an eddy current probe has at least one electrical coil therein. When a signal, i.e. an alternating current, is passed through the coil(s), the coil(s) create a magnetic field. When the eddy current probe is placed adjacent to a conductive surface, the magnetic field interacts with the surface to create circulating eddy currents in the surface. If the surface is generally smooth, the eddy currents may be likened to the circular ripples in a pond after a rock has been dropped in the pond. The eddy currents, however, are generated repeatedly, and cyclically, so long as the signal is provided to the eddy current probe. More specifically, the characteristics of the eddy currents are tied to the characteristics, e.g. frequency, magnitude, phase, etc., of the signal. When there is a defect in the surface or near the surface (hereinafter “at” the surface), the pattern of the eddy currents on the surface is disturbed. By measuring the characteristics of the disturbed eddy current waves, the nature of the defect may be determined.
One type of eddy current probe is identified as a “+Point Probe,” or a “X coil probe.” A “+Point Probe” includes two conductive coils disposed in two generally perpendicular planes within a probe body (thus the “+” or the “X” in the name). Another type of eddy current probe is identified, colloquially, as a “pancake” probe wherein the two coils are stacked on top of each other, or where coils are disposed side-by-side. Of these configurations, the “+Point Probe” is preferred. The “+Point Probe” may be used in one of two modes; a “driver pick-up” mode and an “impedance” mode. In both modes the probe is used to create the eddy currents and to detect disturbances therein. By way of analogy, this is similar to shining a flashlight on a sheet of aluminum foil; where the foil is smooth, the light reflects without disturbance, but, where there is a crease, the light is distorted. In the disclosed method, however, the eddy current probe acts as both the flashlight, creating the light/electromagnetic waves, and the eyes, detecting the defect.
In the driver pick-up mode, the input signal is applied to one of the two coils. This coil creates a magnetic field which, in turn, produces eddy currents in an adjacent surface. The eddy currents also create a magnetic field which may effect the second coil. More specifically, a generally defect free surface will not produce a significant response in the second coil. If a defect exists at the surface, however, an abnormal magnetic field is created and can be detected by the second coil. Due to the interplay between the magnetic fields, in this configuration, the eddy current probe has a greater sensitivity to defects that extend at an angle to the planes of the coils.
In the impedance mode a signal is applied to both coils. Each coil creates a magnetic field and those magnetic fields create eddy currents in an adjacent surface. Further, the impedance created in each coil may be compared to the impedance in the other coil. When the probe is disposed over a generally defect free surface, the impedance in both coils is substantially the same. That is, where there is no defect, the field created by the eddy currents are substantially constant, therefore there is an equal feedback to both coils. A defect, however, disturbs the magnetic field created by the eddy currents and creates more impedance in one of the two coils. By comparing the impedance of the two coils, the defect may be identified. Due to the interplay between the magnetic fields, in this configuration, the eddy current probe has a greater sensitivity to defects that extend within, or parallel to, the planes of the coils.
Thus, an eddy current probe may be used in at least one of two configurations. These two configurations are each likely to detect defects in different planes, either aligned with the plane of a coil or angled relative to the plane of a coil. Thus, the typical method of using an eddy current probe requires the inspection sled to perform two passes over each inspection area; one pass with the eddy current probes in the driver pick-up mode and another pass with the eddy current probes in the impedance mode. This process is expensive and time consuming.